Size of specimens — practice question

The diagram depicts a flatworm that has been enlarged by $\times 100$. The length indicated is $50\,\text{mm}$. What is the flatworm's actual size?

• A$0.5\,\mu\text{m}$
• B$5\,\mu\text{m}$
• C$50\,\mu\text{m}$
• D$500\,\mu\text{m}$